Brillouin flow electron gun



Dec.` 17, 1957 c. K. BIRDSALL BRILLOUIN FLow ELEcTRoN GUN 2 Sheets-Sheet 1 Filed April 26, 1954 De@ 17, 1957 gc. K. BIRDsALL 2,817,035

BRILLOUIN FLow ELEcTRoN GUN Filed April 2e, 1954 z sheets-sheet 2 U\ www@ BRILLOUIN FLOW ELECTRON GUN Charles K. Birdsall, Venice, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Application April 26, 1954, Serial No. 425,391

2 Claims. (Cl. 313-85) This invention relates to electron guns and, more particularly to an electron gun for producing a solid, cylindrical Brillouin flow of electrons having a substantially constant diameter for use in traveling-wave type tubes.

In electron stream-type tubes requiring the use of electron streams having a constant diameter, two different magnetic focusing devices are presently in general use. In one instance, the electron flow produced is called confined ow and in another it is called Brillouin flow. Confined liow is produced simply by subjecting a stream of electrons to an extremely strong axial magnetic iield called an infinite field. This infinite field may generally be reduced by more than fifty percent by employing a Brillouin ilow of electrons. A typical Brillouin flow electron gun comprises a concave cathode, a plurality of focusing electrodes, a magnetic solenoid and several magnetic shielding members. The rules that govern the design and the magnitudes of the operating potentials of these elements relate generally to the physical state of the electron stream as it is introduced into the magnetic field, and to the particular conguration of the field. The stream normally must have no rotational velocity before it is introduced into the magnetic iield which requires that the cathode be thoroughly shielded magnetically. It is also necessary to introduce the stream into the magnetic iield at a certain critical angle which is dependent on the tield configuration. After the stream is introduced into the field, it is also necessary to converge the stream to a certain critical radius, called the equilibrium radius, and at the same time, reduce any radial components of velocity or acceleration to zero at the point along the stream where the equilibrium radius is reached. These requirements make Brillouin ow electron gun construction extremely difficult in that high precision must be maintained in their manufacture and assembly; moreover, delicate operational voltage adjustments add further to the disadvantages accompanying the employment of a typical present day Brillouin flow gun.

By practicing the present invention, a constant diameter Brillouin-how of electrons may be produced by allowing an electron stream to experience a normal space charge spread in a uniform axial magnetic field in conjunction with an electron lens to stop the expansion of the stream at its equilibrium radius. The electron gun of the present invention thus eliminates the complex shielding structures and critical spacing that is generally necessary in prior Brillouin iiow guns.

The structure of the present invention comprises a cathode for producing a thermionic emission of electrons, means for directing the electrons along a predetermined path in the form of an expanding stream and means for maintaining either an electrostatic or av magnetostatic electron lens along the path to converge the` stream to its equilibrium radius at a predetermined point along the path to` reduce any radial velocity components of the stream'. to. zero atthat point. whereby the Brillouin flow of electrons having a substantially constant diameter is produced,

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The employment of the Brillouin ow electron gun of the present invention is more desirable than a comparable electron gun which produces a confined ow since the permissible reduction in axial ux density, which accompanies its employment, makes a reduction in size of the solenoid structure allowable and reduces the operating power loss. A further advantage of the disclosed Brillouin tlow electron gun is that the use of magnetic shielding structure is not required. The electron gun of the present invention may be constructed of known materials by known methods without undue regard to dimensional accuracy in manufacture or to spacing accuracy in assembly. lt is also an advantage of the present invention that, unlike present day Brillouin flow guns, the operating voltages employed need not be adjusted to any particularly critical value.

It is, therefore, an object of this invention to provide an improved electron gun for producing a Brillouin ow of electrons of substantially constant diameter particularly adapted to traveling-wave amplilier tubes.

Another object of the invention is to provide an improved method of producing a solid, cylindrical Brillouin flow of electrons having a substantially constant diameter.

It is a further object of the invention to provide a combination of structures that need not be precision made for producing a solid cylindrical Brillouin ilow of electrons having a substantially constant diameter.

It is another object of the invention to provide an improved Brillouin llow electron gun.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

Fig. l is a diagrammatic sectional view of an embodiment of the invention in an electron stream-type travelingwave amplifier tube.

Fig. 2 is a sectional view of the electron gun of Fig. 1 illustrating a representative electric field pattern and electron stream configuration normally associated therewith.

Fig. 3 is a sectional view of an alternate embodiment of the electron gun of the present invention employing a magnetic lens showing a representative configuration of the electron stream shape normally associated therewith.

Fig. 4 is a graph of the change in axial magnetic flux density linking an electron stream path shown in Fig. 3.

Referring now to the drawing, there is shown in Fig. l an embodiment of the invention incorporated in a traveling-wave amplifier tube 10. This tube includes an input matching cavity 12 having a coaxial input cable 14 and an output matching cavity 16 with a coaxial output cable 18. An envelope 20, which provides the evacuated chamber of traveling-Wave tube 10, consists of a long cylindrical struct-ure which has an enlarged portion at the left extremity as illustrated in the drawing.

Within the enlarged portion at the left extremity, there is located an electron gun 22 for developing a Brillouin tlow of electrons in accordance with the present invention. Gun 22 comprises a cathode 24 with a heater 26, an accelerating anode 28, and a focusing electrode 30. Heater 26 is connected across a source of potential, such as battery 32, the negative terminal of which may be connected to a cathode Z4 in addition to being referenced to ground, as shown. Anode 28 and electrode 30 are connected to adjustable contact arms 34 and 36, respectively, of a source of potential 38, the negative side of which 1s referenced to ground. A potential of the order of 800 volts with respect to ground is representative of the'potential normally applied to anode 28 and electrode 1s normally maintained at a potential 100 to 300 volts lower than that of anode 28.

Cathode 24 comprises a metallic cylinder 40, having an appendage 42 at the end nearest electrode 28, which has an electron emissive coating 44 disposed on its inner surface. Appendage 42 has an aperture defined by an internal surface 46 which is made thermionically emissive by coating 44.

Proceeding along from electron gun 22 in the direction of ow of the electron stream produced thereby, there are positioned successively about the path of the stream. a matching ferrule 48 connected by a lead 50 to a helix .52 which is, in turn, connected by a lead 54 to a matchlng ferrule 56. A collector 58 is positioned at the end of the path so as to intercept and collect the stream electrons.

Helix 52, which serves as the slow-wave circuit for traveling-wave tube 10, is fabricated from a material such as tungsten so that it retains its form, especially with respect to its pitch and diameter. An adjustable or varlable voltage is applied to helix 52 which produces the static electric potential throughout the active length of envelope 20 through which the electron stream flows and, hence, determines the velocity of the stream. This voltage is of the order of 1000 volts with respect to ground and is impressed on helix 52 by means of a connectlon from ferrule 48 to an adjustable tap 60 on source 38.

Due to the fact that traveling-wave amplifiers are broad-band amplifiers, it is diicult to obtain a proper impedance matching for all frequencies at the output. Accordingly, a resistive coating 76, which may be of carbon black, is applied on the outside of envelope 20 about the center turns of helix 52 for the purpose of attenuating waves which may be reected from the output end of tube.

After traversing matching ferrule 48, helix 5,2, and matching ferrule 56, the stream electrons are intercepted by 'collector 58. A potential of the order of 30 volts posltive with respect to that applied to the helix is applied to collector 58 in order to prevent secondary electrons, which may be produced by the stream electrons impinging on its surface, from reaching helix S2 or ferrule 56. This potential is applied by means of a connection from collector 58 to the positive terminal of source 38.

A solenoid 78 is axially positioned symmetrically about the complete length of envelope 20. An appropriate direct current is maintained in solenoid 78 by means of a potential source, such as a battery 80, so as to produce an axial magnetic field of the order of 500 gauss running along the length of the traveling-wave tube and completely immersing the electron gun 22.

In the operation of amplifier 10, an input signal to be amplified is applied through input coaxial cable 14 to input cavity 12 to launch a traveling-wave along the helix 52. Interaction between the electron stream and this traveling-wave results in a transfer of energy from the stream to the wave, causing it to grow or increase in magnitude. At the end of helix 52, the amplified electromagnetic wave, in flowing along lead 54 connecting helix 52 to ferrule 56, excites an electric field in cavity 16 to provide an amplified output signal which is available through output coaxial cable 18.

The operation of the electron gun 22 of the present invention is more adequately explained in connection with Fig. 2 wherein the electron gun 22 is shown with a representative configuration of the electric fields associated therewith.

In its operation, cathode 24 provides a highly focused dense source of electrons in that nearly all the electron emission is from the inner surface 46 of the aperture in appendage 42. The electric field established by means of the potential applied to anode 28 accelerates the electrons along the path on the longitudinal axis of envelope 20. Between anode 28 and focusing electrode 30, however, there is a decelerating electric field produced by the potential impressed on electrode 30 which is several hundred volts negative with respect to that applied to anode 28. In this decelerating electric field the space charge forces of the electron stream overcome the constraining effect of the axial magnetic field and cause it to expand outwards. In expanding outwards, the axial magnetic field imparts a rotational component of velocity to the stream electrons.

Subsequent to this expansion and rotation, the electron stream passes through an electron lens within the aperture in focusing electrode 30 where the radial velocities are substantially reduced to zero leaving only the rotational and axial components. In accordance with the present invention, these rotational and axial components of velocity are basically such that there is no net radial movement of the electrons as the stream is directed along the path through matching ferrule 48, helix 52 and, matching ferrule 56. That is, the inward forces due to the rotation of the electrons in the magnetic field is equal and opposite to the outward centrifugal force and the space charge forces of the stream.

Thus it is seen that Brillouin How is produced when the sum of the radial forces on the electrons constituting the stream becomes continuously equal to zero. More specifically, it can be shown that for this condition to exist, the following relationship must be satisfied at any point along the stream:

w1, is called the Larmor frequency w, is called the cyclotron frequency wp is called the plasma frequency B is the axial flux density of the magnetic field e a negative number, is the charge of an electron m is the mass of an electron I is the current in the electron stream e is the permittivity of free space ris the equilibrium radius of the stream V is the stream voltage The cyclotron frequency is determined independently of the plasma frequency according to a theorem called Buschs theorem from which the following expression is where pc is defined as the quantity of magnetic ux linking the helical path of an electron at one point along a stream and xp is defined as the flux linking the helical path at another point further along the stream. When the axial fiux density is uniform along the path, Equawhere rc is the radius of the electron path at the first mentioned point and r is the radius of the path at the second mentioned point.

`It is desirable to make the cyclotron frequency equal to the Larmor frequency in producing Brillouin flow because when `this relationship exists, the electron stream may be kept flowing at a constant diameter with a minimum magnetic tux density. This relationship may be derived in theory by writing the force equation for radial wherein 'r' is the radial acceleration of an electron, Er is the radial electron eld intensity due to the existence of the stream electrons, and r is the radius of the path of the electron. The value of the cyclotron frequency for the minimum value of the radial force, mi", may be determined from this Equation 6. Plotting the radial force as a function of the cyclotron frequency, it can be seen that the characteristic curve is a parabola having a minimum point at its vertex where the cyclotron and Larmor frequencies are equal and that measurable variations in the cyclotron frequency from the Larmor frequency cause only negligible changes in the radial force.

The cyclotron frequency may be easily made approximately equal to the Larmor frequency by making the equilibrium radius of the stream larger than the radius of the electron stream at its origin. In the particular case at hand, the radius of the aperture of cathode 24 is made substantially smaller than the equilibrium radius of the electron stream. It is evident that this dimensional relationship should be maintained when, in Equation 5, the rst mentioned point is taken to be at the aperture of cathode 22 and the second point where there is equilibrium. Using rc and r as the aperture radius and equilibrium radius, respectively, it is apparent from Equation 5 that when Hence, for satisfactory operation, the stream need only expand three or four times the diameter of the cathode aperture.

Cathode 24 need not be constructed in the manner shown, but any conventional cathode may be used. It is, however, desirable to keep the diameter of the electron stream small at its origin, according to the aforementioned design techniques, so as to allow the stream to expand to two to four times its original diameter. However, in order to avoid the backward wave selfoscillation range encountered at higher frequencies, it

is necessary to keep the overall helix and stream diameters small. This imposes a limitation opposite in effect to that of making the cyclotron frequency equal to the Larmor frequency, i. e. the stream equilibrium radius should be small. Since cathode 24 is of the type that provides an appreciable current from a small diameter source, it may be employed and still allow for the necessary expansion to the equilibrium diameter of the stream.

Depending upon the specific structure of electron gun 22 and the associated circuit parameters, the electron stream produced by cathode 24 will expand to its equilibrium radius at some point along the stream within the eld produced by solenoid 78. Although numerous precautions are taken, a stream electron, at the initial point where the equilibrium radius is reached, will diverge beyond its equilibrium radius because its radial velocity is not zero at that point. The stream diverges and subsequently converges after reaching a maximum radius. These variations then recur periodically throughout the complete length of the stream beyond the point that the equilibrium radius is reached. Compensation for the variations in gun 22 is accomplished by means of the electron lens provided by focusing electrode 30 which is employed to converge the stream to the equilibrium radius and at the same time reduce the radial velocities of the stream electrons to zero.

Referring to Fig. 3 there is shown an embodiment of the present invention wherein a magnetostatic lens is employed to replace the electrostatic lens provided by focusing electrode 30. In the electron gun of Fig. 3, a magnetostatic lens is provided by a solenoid 86. An appropriate direct current is maintained in solenoid 86 by means of a potential source 80. The high liux density which is produced by solenoid 86 passes through the electron stream in the region coextensive with an annular gap 8S in the magnetic shielding structure 90 disposed about the solenoid 86. A representative variation of relative ux density with distance on each side of gap 88 in structure 90 is illustrated in Fig. 4. The structure 90 is maintained at an appropriate potential intermediate that of anode 28 and ferrule 48 by a connection to a suitable tap on battery 3S.

Thus in the electron gun of the present invention, the solenoid 86 provides a magnetostatic electron lens along the electron stream developed by gun 22. The increased magnetic field provided by lens 86 increases the inward radial acceleration forces on the stream electrons in a manner to compensate for any unbalanced outward radial acceleration.

What is claimed is:

1. An electron gun for producing a solid, cylindrical Brillouin flow of electrons having a substantially constant predetermined final cross-sectional area along a predetermined path, said gun comprising a source of electrons disposed on said path for producing an electron stream having a predetermined initial cross-sectional area; means for producing an axial magnetic field along said path extending through said source; and means including an electron lens for directing electrons away from said source at a velocity to allow the space charge of said stream to expand the volume occupied by said stream outwards to a volume having a predetermined final circular cross-sectional area several times greater than said predetermined initial cross-sectional area of said electron stream whereby the outward expansion of said electrons in said axial magnetic field causes them to rotate about the longitudinal axis of said path.

2. An electron gun for producing a solid, cylindrical, Brillouin tiow of electrons having a substantially constant predetermined nal cross-sectional area along a predetermined path, said gun comprising a source of electrons disposed on said path for producing an electron stream, said stream having a predetermined initial cross-sectional area; means for producing an axial magnetic eld along said path extending through said source; means for accelerating said electrons away from said source along a portion of said path at a velocity to allow the space charge of said stream to expand said stream outwards in said axial magnetic iield whereby said stream electrons are caused to rotate about the longitudinal axis of said path; and means for producing an electron lens disposed subsequent to said portion of said path at a point where the radial forces acting upon said stream electrons are in equilibrium to reduce the radial velocity components of said stream electrons to zero at said point, the crosssectional area of said stream being equal at said point to said predetermined nal cross-sectional area.

References Cited in the file of this patent UNTTED STATES PATENTS 2,400,331 Bachman May 14, 1946 2,424,965 Brillouin Aug. 5, 1947 2,632,130 Hull Mar. 17, 1953 2,687,490 Rich et al. Aug. 24, 1954 2,741,718 Wang Apr. 10, 1956 

